5.0 RESULTS AND DISCUSSION
The experimental results of the study are presented in Tables 3 through 8.
Tables 3 and 4 present the ESD surface voltage measurement data for the removal of urethane and parylene conformal coatings respectively for wheat starch media. The ESD surface voltages are reported as minimum, maximum and average measurement values. The data for site 5 shows the surface voltages measured without the use of a point ionizer for ESD control.
The last column reports the electrostatic field within the work chamber measured by the static sensor.
Tables 5 through 7 present the ESD surface voltage measurement data for sodium bicarbonate, plastic bead and glass bead medias respectively.
Table 8 is a summary of ESD surface voltage measurements for all four medias evaluated showing the average surface voltages for sites 1 through 4.
In evaluating the effectiveness of the micro blasting technique to remove the conformal coatings, it became evident that the removability is a strong function of several variables including the type of the coating, it's thickness and the characteristics of a blast media. The results showed that the urethane conformal coating was more difficult to remove than the parylene for all four medias. The urethane conformal coating had tendency to peel off while the parylene conformal coatings eroded.
The cutting performance of the various blast medias showed mixed results. The microblaster parameters were optimized for media flow rate and gas pressure, however the effect of other factors such as nozzle parameters (orifice size and shape) and particle size variation for each media were not considered in this study. The main area of focus of this experimental study was ESD voltage generation during microblasting. The following observations describe the characteristics of the various medias evaluated.
- Wheat Starch
Crystal Mark's wheat starch (carbo-blast) media is specially developed for conformal coating removal in ESD sensitive applications. This media is water soluble, biodegradable and considered environmentally friendly. It had an average particle size of 120 micron and showed good flow without any nozzle clogging problems.
Both types of coatings could be easily removed relatively quickly without any visible damage or penetration into the PWB surface. The total time of removal at each site was less than 20 seconds for urethane and less than 10 seconds for parylene. In general, the removal was quicker and more uniform in the pad/trace areas compared to the solder joint areas. This media generated an average ESD surface voltage ranging from 16-66 volts for urethane and less than 30 volts for the removal of parylene.
The data from the voltage measurements when ESD control was not used showed interesting results proving the advantage of using a point ionizer. These measurements were taken at the surface mount device (SMD) pad area (site 5). The ESD surface voltage spiked to an average of 1529 volts for urethane and -445 volts for parylene conformal coating.
- Sodium Bicarbonate
The sodium bicarbonate media from Crystal Mark with 75 micron average particle size was used for this study. This media is water soluble, non-toxic and environmentally acceptable.
The cutting characteristics of sodium bicarbonate were good for both types of the conformal coatings. The media flowed well through the microblaster without any nozzle clogging problems. Overall, the coating removal rate was somewhat slower for both types of coatings (approximately 30 seconds) as compared to wheat starch media. The removal time was higher for the solder joint areas (site 3) on urethane attributable to higher coating thickness in those areas.
Sodium bicarbonate was effective in removing both conformal coatings, however, it generated higher levels of surface voltages compared to wheat starch. The average surface voltage ranged from -100 to -150 volts for urethane and spiked to -600 volts for parylene conformal coating. Without the use of a point ionizer, ESD surface voltages spiked to an average of 1400 volts for urethane and 3300 volts for parylene removal.
- Plastic Bead
The plastic bead (plastic-blast) media from Crystal Mark had an average particle size of 120 micron. It is commercially used for deflashing plastic electronic component leadframes. This media is not biodegradable.
Overall, this media performed well in removing both types of conformal coatings without any nozzle clogging problems or PWB surface damage. The conformal coating removal times were comparable to those for wheat starch and sodium bicarbonate.
The plastic bead media also generated higher surface voltages during parylene removal compared to urethane. The average ESD surface voltage ranged from -20 to -200 volts for urethane and 25 to -560 volts for the removal of parylene. These values peaked to 3000 volts when the ESD control device was not used.
- Glass Bead
The glass bead media with 44 micron average particle size from Crystal Mark was used. This media is considered environmentally safe.
The glass bead media from Crystal Mark did not exhibit acceptable cutting characteristics. The urethane conformal coating on trace areas (Site 2) could not be completely removed after an extended period of blasting. However, for parylene, there were signs of solder joint erosion and board surface damage in some areas.
In addition, this media generated unacceptable levels of surface voltages in both types of conformal coatings. This was observed at low media flow rate and pressure conditions. The average ESD surface voltages spiked to 682 volts and -878 volts for urethane and parylene respectively.
6.0 SUMMARY AND CONCLUSIONS
An experimental evaluation was performed to determine the effectiveness of a micro abrasive blasting technique for the removal of the conformal coatings. The two main areas of concern were ESD voltage generation at the surface and physical damage to the PWAs. Test PWAs used in this evaluation were conformally coated with urethane and parylene. The SWAM BLASTER coating removal system was used with four blast medias including wheat starch, sodium bicarbonate, plastic beads and glass beads.
The conclusions from this evaluation are summarized below:
- The SWAM BLSATER coating removal system was effective in removing both coatings. Overall, urethane coatings had a slower removal rate than parylene.
- Wheat starch demonstrated the best overall cutting characteristics for urethane and parylene coatings indicating that micro abrasive blasting technique may be acceptable for space applications.
- Glass beads showed the worst results for coating removal, high ESD surface voltage generation, and board damage. Sodium bicarbonate and plastic media showed mixed results.
- The ESD surface voltages generated using wheat starch were less than 100 volts and were the lowest levels among all four medias. Sodium bicarbonate, plastic beads, and glass beads generated ESD surface voltage between 100 and1000 volts.
- The Crystal Mark point ionizer was very effective in reducing the surface voltages generated. The ESD surface voltage measured in excess of 3000 volts without the use of a point ionizer.
Before any commercially available medias are used, a thorough evaluation must be performed to assess the effects on the PWB surface and ESD damage to the parts and electrical circuits on a PWA.
There are a number of other variables which would strongly influence the performance of the micro blasting process which were not investigated in this study.
- Other media types
- Various particle sizes
- Relative humidity in the chamber
- Nozzle parameters (size and shape)
- Media residue and its effect on cleanliness level and solderability
- Electrical functionality characterization (for static sensitive devices)
- Other PWB material types
- Additional ESD elimination techniques
The above areas need further research to address the concerns of the micro abrasive blasting technique in the removal of the conformal coatings.
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